1. Field of the invention
The present invention is related to the field of cameras that include optical filters with spatially adjustable variable optical density.
2. Description of the related art
Cameras using detector arrays, such as CCD arrays at the focal plane suffer from the drawback that the dynamic range of the detectors is limited to a window of values of intensity. The window can be adjusted up or down. The adjustment is typically optimized by setting the window to match the average illumination level.
No matter where the intensity window is set, illumination with intensity outside the window will not be imaged correctly. Points of lesser illumination will appear as having a uniform minimum threshold value, losing all detail below that. Points of higher illumination will do the same with a maximum threshold value. In addition some of them may xe2x80x9cspill overxe2x80x9d to adjacent neighboring elements that are less illuminated, causing xe2x80x9cbloomingxe2x80x9d or xe2x80x9csmearingxe2x80x9d.
A solution is to use a spatially adjustable variable density light filter in conjunction with the camera. A specific application is taught in U.S. Pat. No. 5,541,705 to Kan et al., reproduced in this document as FIG. 1. A receiving camera 20 has an objective lens 22 and a CCD array 24 at the focal plane of the objective lens. The filter is made from a liquid crystal light valve (LCLV) 30, that is placed in the path of the beam with the image and between a polarizer 32 and an analyzer 34.
A main lens 40 and a relay lens 42 focus the image successively through the filter, a second LCLV 46, and then onto the camera. A scanner control 50 scans the scene, and accordingly provides a signal. The scanner signal goes to an optional brightness information recorder or transmitter 52. The scanner signal also goes to LCLV display driver 54, that drives both the LCLVs. Each LCLV is made from a two dimensional array of elements whose optical density is individually controllable. This way they can make local bright spots dimmer, bringing them within the window of values of the dynamic range.
A problem identified by the inventors in the design of such systems is that the LCLV itself ordinarily tends to cast an undesirable image on the CCD array itself. The prior art has avoided this problem by placing the LCLV at an effectively infinite optical distance from the CCD array. For example, in FIG. 1, LCLV 30 is at the focal plane of lens 40.
This solution has been unsatisfactory. Adding lenses 40 and 42 makes the optical path of the whole system longer, and adds to the cost.
It is desired to exploit the advantages of variable density filters without the cost of additional lenses, or requiring a longer optical path.
The present invention overcomes these problems and limitations of the prior art.
The invention provides a camera that includes an array of optical detectors and an optical filter placed adjacent the array. This way the filter does not cast an objectionable image. This placing eliminates the need for using the additional lenses, and for dedicating the optical path length that they require.
The filter includes spatial sections of individually adjustable optical density. The optical densities are controlled by individual control signals. The optical detectors output detection signals that are used to form the control signals. The camera outputs the combined control signals and detection signals, thus effectively providing a larger dynamic range than would be provided by the detection signals of the detectors alone.
The camera is preferably provided with an objective lens that defines a focal plane. The detector array is placed at the focal plane. The camera can include a housing to form a stand alone unit, that does not need to be larger than an equivalent prior art camera.
The detectors can have different responses to different wavelengths. As such, color specific effects are attained.
The invention further provides a method for controlling the filter. The detection signals are scanned to ensure that a predefined filtering condition is met. If not, control signals are iteratively generated and applied to the filter until the condition has been met. Then the detection signals are combined with the control signals to form composite signals that encode the intensity distribution of the beam. Control methods ensure greater dynamic range and accuracy. Image processing methods enhance viewability.
The invention can be used advantageously in video cameras and still image cameras, such as those in digital copiers. Another advantage of the invention is that it permits the use of sensors with very low dynamic range.